1. Field of the Invention
The present invention relates to a measurement apparatus.
2. Description of the Related Art
Photoacoustic tomography (“PAT”) is a known measurement technique of a spectroscopic (or attenuation) characteristic in biological tissue. PAT is a method that utilizes a difference in absorption of light energy as between a target region, such as a tumor (object), and background tissue (scattering medium), and receives through an elastic wave detector an elastic wave (typically an ultrasonic wave) that occurs as a result of the target region absorbing irradiated light energy and instantly expanding (U.S. Pat. No. 6,738,653).
When a stress confinement condition is met in which a pulse width of the light irradiated upon the target object from the light source is shorter than the stress relaxation time, the elastic wave has a sound pressure expressed by the following equation using a distance “z” from the light source, which is proportional to the light intensity and the absorption coefficient at a position of the target region that is a sound source. Here, P(z) is a pressure of the elastic wave at distance z, Γ is a Grüneisen coefficient (heat-acoustic conversion efficiency), and μa(z) is an absorption coefficient at distance z. Φ(z) is the light intensity at distance z.
                              P          ⁡                      (            z            )                          =                              1            2                    ⁢          Γ          ⁢                                          ⁢                                    μ              a                        ⁡                          (              z              )                                ⁢                      Φ            ⁡                          (              z              )                                                          Equation        ⁢                                  ⁢        1            
In the field of diffused optical tomography (“DOT”), it is known that the light intensity Φ(z) in the scattering medium follows the following equation when absorption and scattering are viewed macroscopically. DOT is a measurement technique that introduces near-infrared light into the scattering medium and detects the diffused light. Here, “A” is a proportionality constant, and μeff is an effective attenuation coefficient.Φ(z)=AΦ0exp(−μeffz)  Equation 2
FIG. 1 shows a sound pressure distribution of an elastic wave when there is a heterogeneous object (for example, having a higher absorption coefficient) in a scattering medium that can be considered homogeneous. An object signal region H has a bipolar-shaped waveform similar to a letter N, and a background signal region B—the region other than the object signal region H—exponentially changes (Valery V. Tuchin, Handbook of Optical Biomedical Diagnostics, pp. 585-646; Rinat O. Esenaliev et al., “Sensitivity of Laser Opto-Acoustic Imaging in Detection of Small Deeply Embedded Tumors,” IEEE Journal Of Selected Topics In Quantum Electronics, Vol. 5, No. 4, pp. 981-988 (1999)). M. Xu et al., “Photoacoustic Imaging in Biomedicine,” Review of Scientific Instruments 77, 041101 (2006), and A. Oraevsky et al., “Measurement of Tissue Optical Properties by Time-Resolved Detection of Laser-Induced Transient Stress,” Appl. Opt., Vol. 36, No. 1, pp. 402-415 (1997) propose to use Equation 2 to fit the background signal component that indicates the homogeneous part and to calculate an attenuation coefficient.
The spectroscopic (or attenuation) characteristic is affected by both the absorption (spectroscopic) characteristic and the scattering (spectroscopic) characteristic in the scattering medium. Acquisition of the absorption characteristic is desired because an amount of each of the components of the medium, such as hemoglobin, collagen, and water, can be calculated from the absorption characteristic of the light. However, in the attenuation coefficient, the absorption characteristic and the scattering characteristic are not separated and the absorption characteristic cannot be precisely evaluated. Conventional PAT cannot obtain the absorption characteristic. Moreover, unlike the proposals of Xu and Oraevsky, it is important to evaluate a characteristic of the object in addition to a calculation of the attenuation coefficient of the background signal.